Method of and an apparatus for bulge forming



July 14, 1970 ICHIZO OTODA 3,520,163

METHOD OF AND AN APPARATUS FOR BULGE FORMING Filed NOV. 24, 1967 7 sheets sheet 1 J mam INVENTOR July 14, 1970 ICHIZO OTODA 3,520,163

METHOD OF AND AN APPARATUS FOR BULGE FORMING Filed NOV. 24, 1967 7 Sheets-5heet :2

FIG. 7a

PRIOR ART July 197G lcH zo QTQDA 3,520,163

OF AND AN APPARATUS FOR BULGE FORMING Filed NOV. 24 1957 '7 Sheets-5hei L Fig 5 PRIOR ART FIG. 5 PRIOR ART FIG. I30 FIG. l3b

July 14, 1970 lCHiZO OTODA 3,

METHOD OF AND AN APPARATUS FOR BULGE FORMNG Filed Nov. 24, 1967 7 Sheets-Sheet 4 I b Wag! July 14, 1970 ICHIZO OTODA 3,

METHOD OF AND AN APPARATUS FOR BULGE FORMING Filed NOV. 24, 1967 '7 Sheets-Sheet 5 FIG /5b /a 49 /f /d /e /0 60 F762 I60 F/G. I60

Juiy 14, 1970 gl-uzb OTODA 3,520,i63

METHOD OF AND AN APPARATUS FOR BULGE FORMING Filed Nov. 24, 196'? 7 Sheets*Sheet 6 FIG. /8

July 14, 1970 iCHIZO OTODA 3,520,163

METHOD OF AND AN APPARATUS FOR BULGE FORMING Filed NOV. 24, 1.967 7 Sheets-Sheet 5 F G 22 42 38 /8/ '/6/a 50 /7 /6 /0 /8 39 United States Patent US. Cl. 7259 8 Claims ABS CT OF THE DISCLOSURE A method of and an apparatus for bulge forming of a thin tubular material are provided, wherein an elastic bulge forming punch of a tubular shape having an outer contour corresponding to a required contour of a formed product is inserted into the tubular material and is hydraulically expanded by an internal pressure applied therein to exert working force on the tubular material, whereby a conventional outer mould or die is excluded and the possibility of application of bulge forming is greatly increased in broadened fields.

In the conventional bulge forming of a thin tubular material which is to be formed into a convex and concave contour, a split outer mould or die having a moulding surface complementally corresponding to the convex and concave contour of the formed product must be arranged onto the tubular material.

However, such an outer mould brings about not only economic drawbacks that the cost for manufacturing the mould is relatively high and the process of bulge forming becomes much difiicult and complicated requiring more time and labour but also the relatively restricted application of the bulge forming since an outer mould which has a larger bulk than the product is sometimes not employable in a narrow space because of the existence of other members. The scarring of the outer surface of a product is also sometimes a serious problem.

As the known methods of bulge forming, there are three methods, that are (i) Liquid bulging, (ii) Electrohydraulic forming and ((iii) Rubber punch bulging, and in all of these methods an outer mould or die is necessary. In other words, in all of these methods, a die which has an inside moulding surface complementally corresponding to the convex and concave contour of a formed prod net and is split to complementary half die portions is prepared, and after a thin tubular material has been placed in the die, an internal pressure is applied to the inside of the thin tubular material by means of hydraulic fluid or an elastic material so that the thin tubular material is expanded to be in firm contact with the inside surface of the die and is correspondingly formed.

For the illustrative purpose of the conventional methods, an example of bulge forming of a thin tubular material 6' as shown in FIGS. 1a and lb into a convex and concave contour as shown in FIGS. 2a and 2b will be described in the following.

(i) Liquid bulging A bulge forming apparatus for use in this method is shown in FIGS. 5a and 5b, wherein a split die 2 having an inside moulding surface 2a complementally corresponding to the convex and concave contour of the formed product is held in a die holder ring 3 and is seated on a base plate 7. In the die is placed a thin tubular material 6 with both ends thereof being plugged 'by packings 4 and '5, wherein the packing 4 is provided with a central bore for leading hydarulic fluid. On the packing 4 is mounted a block 1' having a central bore 1a, through which a high pressure liquid is supplied. Thus the thin tubular material 6' is expanded by the internal pressure of the liquid and is bulged to follow the inside convex and concave surface 2a of the die.

(ii) Electrohydraulic forming This method has been developed in recent years and the products formed in this method are now on the market. This method of forming utilizes the impact force of the electric discharging and is practised by a bulge forming device as shown in FIG. 6, wherein a split die 2 has an inside moulding surface 2a complementally corresponding to the convex and concave contour of the formed product and is held in a die holder ring 3. In the die is placed a thin tubular material 6 with its bottom end being plugged by a packing 5. Electrodes 8 and 8 are opposedly arranged within the tubular material, the inside of which is then filled with water 9 as shown in FIG. 6. When the electric discharging is generated between the electrodes 8 and 8', the impact force is transmitted via the water 9 to the inner wall of the thin tubular material 6', which is thus expanded and formed to follow the inside convex and concave surface of the die. Passages 2b, 3a are provided for releasing air.

((iii) Rubber punch bulging This method utilizes the elasticity of rubber and is practised by a bulge forming device as shown in FIGS. 7a and 7b, wherein a split die 2 and a die holder ring 3 of similar kind as those in FIG. 5 are arranged on a base plate 7. A thin tubular material 6 is placed in the die, and a cylindrical bulge punch 1 of rubber is placed in the thin tubular material. Thereafter, a metal plunger 1 having an outer diameter which is substantially equal to the inner diameter of the thin tubular material 6' is placed on the rubber punch and is applied with compression force, whereby the rubber punch is elastically compressed in axial direction as well as expanded in radial directions, thus in turn expanding the thin tubular material 6' from its inside to make it follow the convex and concave surface 2a of the die 2, as shown in FIG. 8.

With reference to each of the conventional methods described in the above, it will be noted that an outer mould or die is not only indispensable but also must be a split die so that the formed product can be taken out. The split die requires in addition a die holder ring to keep it closed during the bulge forming, which brings about a drawback that the bulge forming device becomes more bulky. As to the pressurizing system, the liquid bulging and the rubber punch bulging are relatively simple, but the electrohydraulic forming necessitates a condenser of a large capacity for electric discharging and also the former two bulging methods have a drawback that because the liquid flows out when the product is taken out, the operation is very difiicult. In case of the rubber punch bulging, a press is necessary to compress the rubber punch and the bulge forming device must be mounted on the press. Assembling and disassembling of the split die for each product is inherently bound with a low efficiency.

The aforementioned drawbacks of the known methods of bulge forming can be avoided by an improved method of bulge forming according to the present invention,

wherein the method is characterized by the steps of inserting an elastic bulge forming punch in the shape of a thick tube having an outer contour corresponding to a required contour of a product into a thin tubular material for the product and applying pressure to the inside of said bulge forming punch to expand said bulge forming .3 punch, whereby the thin tubular material is bulged by the force exerted only from its inside with cooperation of no outer mould.

Accordingly, an object of the present invention is to provide a method of bulge forming which is simple and efiicient in process and an apparatus for bulge forming which is simple in structure and easy in operation.

Another object of the present invention is to provide a method of and an apparatus for bulge forming applicable to simultaneous bulge forming at several positions of an elongated thin tubular material onto which no outer mould is applicable.

Still another object of the invention is to provide a method of and an apparatus for bulge forming wherein the bulging force can be varied along the length to be bulged.

Other objects and advantages of the present invention will become apparent from the descriptions of some preferred embodiments which will be made with reference to the accompanying drawings.

FIGS. 1a and 1b are respectively an end view and an axially sectional view of a thin tubular material to be bulged;

FIGS. 2a and 2b are respectively an end view and an axially sectional view of a bulged product;

FIGS. 3a and 3b are respectively an end view and a side view of another elongated thin tubular material to be bulged;

FIGS. 4a and 4b are respectivly an end view and a side view of another elongated bulged product;

FIGS. 5a and 5b are respectively a plane view and an axially sectional view of a conventional liquid bulging apparatus in schematic illustration;

FIG. 6 is an axially sectional view of a conventional electrohydraulic forming apparatus in schematic illustration;

FIGS. 7a and 7b are respectively an end view and an axially sectional view of a conventional rubber punch bulging apparatus in schematic illustration;

FIG. 8 is an axially sectional view of the rubber punch bulging apparatus shown in FIGS. 7a and 7b, where the rubber punch is under compression in schematic illustration;

FIGS. 9a and 9b are respectively an end view and an axially sectional view of a bulge forming apparatus according to the present invention in schematic illustration;

FIGS. 10a, 10b and 100 are respectively an end view, an axially sectional view and a side view of a bulge forming punch used in the bulge forming apparatus shown in FIGS. 9a and 91);

FIGS. 11a and 1119 are respectively an end view and an axially sectional view of an inner punch used in the bulge forming apparatus shown in FIGS. 9a and 9b;

FIGS. 12a and 121) are respectively an end view and an axially sectional view of another bulge forming apparatus according to the present invention in schematic illustration;

FIGS. 13a and 13b are respectively an end view and an axially sectional view of a bulge forming punch used in the bulge forming apparatus shown in FIGS. 12a and 12b;

FIG. 14 is an axially sectional view of still other bulge forming apparatus according to the present invention in schematic illustration;

FIGS. 15a and 15b are respectively an end view and an axially sectional view of a bulge forming punch used in the bulge forming apparatus shown in FIG. 14;

FIGS. 16a and 16b are respectively an axially sectional view and an end view of a stator of a canned motor manufactured according to the present invention in schematic illustration;

FIG. 17 is an axially sectional view of the stator shown in FIGS. 16a and 16b in the state where a stator inner liner is not yet bulged;

FIG. 18 is an axially sectional view of a bulge forming apparatus according to the present invention for forming a central straight portion of the stator inner liner shown in FIGS. 16a and 16b in schematic illustration;

FIG. 19 is an axially sectional view of the partly formed stator formed by the bulge forming apparatus shown in FIG. 18;

FIG. 20 is an axially sectional view of a bulge forming apparatus according to the present invention for forming corrugated portions at both ends of the stator inner liner shown in FIGS. 16a and 16b in schematic illustration;

FIG. 21 is an axially sectional view of another bulge forming apparatus according to the present invention for forming the corrugated portions at both ends of the stator inner liner shown in FIGS. 16a and 16b in schematic illustration; and

FIG. 22 is an axially sectional view of a bulge forming apparatus according to the present invention for forming simultaneously the central straight portion and the corrugated portions at both ends of the stator inner liner shown in FIGS. 16a and 16b in schematic illustration.

The present invention relates to a method of bulge forming wherein a bulge punch in the shape of a thick tube having an outer contour corresponding to the required convex and concave contours of a product is elastically expanded in radial directions to bulge and form a thin tubular material as required without employing any outer mould or die. In the following, several examples of embodiments of the present invention will be described for the illustrative purpose of the invention.

EXAMPLE 1 A bulge forming punch of double structure A bulge forming apparatus embodying the present invention is shown in FIGS. 9a and 9b and a bulge forming punch and an inner punch which are used in the bulge forming apparatus in FIGS. 9a and 9b are shown in FIGS. 10a to 10C and FIGS. 11a, 11b, respectively. The bulge forming punch I is a thick tube being made of a hard elastic material such as rubber and having an outside contour 1a corresponding to the convex and concave contour of a product article and a central bore, as shown in FIGS. 10a to 100. The inner punch which is inserted into the bulge forming punch to prevent the leakage of liquid and to transmit the radial expansion force due to hydraulic pressure to the bulge forming punch is made of a soft eleastic material such as rubber and has a shape of a tube having inwardly folded opposite ends (FIGS. 11b). A punch holder 11 has a central hole 11a for introducing liquid to the inside of the inner punch 10, thus transmitting hydraulic pressure to the inside thereof. The inwardly folded portions 10a of the inner punch 10 are respectively received on reduced shaft portions 11b of the punch holder 11, and opposite end walls 10b of the inner punch are restricted between a flange portion of the punch holder and an end plate 12 supported by a nut 13 screwed onto a threaded end of the punch holder, whereby it is prevented that the inner punch is extended in the axial direction or is subjected to breakage when the hydraulic pressure is applied. Thus, under the application of the hydraulic pressure, the inner punch 10 is expanded only in the radial directions and expands the bulge forming punch 1 radially outward by the force exerted from the inside. The bulge forming punch 1 which is applied with internal pressure via the inner punch 10 and is expanded in radial directions expands in turn a thin tubular material 6' by the convex and concave portion 1a and forms the matrial in the correspondingly bulged convex and concave contour. After the completion of bulge forming, the hydraulic pressure is released, whereby the bulge forming punch 1 and the inner punch 10 elastically contract to their initial configurations. Thereafter, the nut 13 is unfastened and the end plate 12 is removed from the punch holder 11, whereby the bulged product can be readily taken out.

In the experiment, a seam-welded tube material of stainless steel having outer diameter of 46 mm. and thickness of 0.15 mm. was readily formed into a product having the contour as shown in FIGS. 2a and 2b, wherein the outer diameter of the convex portion was 51 mm. and the outer diameter of the concave portion was 49.2 mm. The bulging tools used in the experiment are as shown in FIGS. 9a and 9b and have the following characteristics data: The bulge forming punch 1 is made of a hard polyurethane rubber and has a central hole of 36 mm. and an outer surface incorporating convex portions of 44 mm. outer diameter and concave portions of 42 mm. outer diameter. The inner punch is made of a highnitril rubber and has a cylindrical body of 35.5 mm. outer diameter and 2 mm. thickness, which is transferred to opposite end walls of 3 mm. thickness and then to respective inwardly folded portions 10a of 23.5 mm. inner diameter and 12 mm. axial length. The punch holder 11 is made of steel and has a shaft portion of 28 mm. diameter provided with steppedly reduced portions 11b of 24 mm. diameter to receive thereon the inwardly folded portions 10a of the inner punch. The shaft portion is provided with an oil leading bore 11a of 6 mm. diameter along its axis and a cross bore 11c of 6 mm. diameter traversing the central bore 11a. The end plate 12 is made of steel.

In the experiment, the bulge forming punch 1 and the inner punch 10 were assembled on the punch holder 11, the seam-welded stainless steel tube 6' of 46 mm. outer diameter and 0.15 thickness being mounted on the bulge forming punch 1, and thereafter the end plate 12 was mounted onto the end of the shaft porti n and clamped by the nut 13 as shown in FIG. 9b. The bulge forming tools thus having been assembled, oil pressure of kg./cm. was applied via the oil leading bore 11a of the holder for approximately one second. By the application of this internal pressure, the inner punch 10 and therefore the bulge forming punch 1 were elastically expanded radially outward, thus bulging the thin seamwelded stainless steel tube 6 fitted over the bulge forming punch 1 from its inside radially outward according to the convex and concave contour of the outside surface of the bulge forming punch. The formed product was in the shape as shown in FIGS. 2a and 2b, wherein the outer diameter of the convex portion was 51 mm. and the outer diameter of the concave portion was 49.2

In this condition of bulge forming, the external and internal pressures exerted on the inner punch 10 were respectively about 0.40 kg./mm. and as to the bulge forming punch 1 the internal ressure exerted thereupon was about 0.40 kg./mm. and the external pressure was 0.27 kg./mm. Since these pressures were lower than the elastic limit of the rubber, there occurred no breakage in the inner punch and bulge forming punch made of the rubber. The convex portion of the bulge forming punch elastically reduced its height only by about 10 percent due to the external pressure of 0.27 kg./mm. Therefore, by giving the bulge forming punch a contour which was correspondingly modified for the elastic deformation, highly precise product could be obtained.

EXAMPLE 2 A single bulge forming punch of a simple structure Another bulge forming apparatus embodying the present invention is shown in FIGS. 12a and 12b and a bulge forming punch thereof is shown in FIGS 13a and 13b. The bulge forming punch 1 in FIGS. 13a and 13b is also made of a hard eleastic material such as rubber and is also a thick tube having an outside contour 1a corresponding to the convex and concave contour of a product article and a central bore as shown FIGS. 13a and 13b. The bulge forming punch 1 in this example has in addition annular lips 112 at its opposite ends, which serve to prevent leakage of high pressure liquid. The opposite ends of the bulge forming punch 1 are received in counterbores of two end plates 11 and 12, the diameter of each counterbore being slightly smaller than the peripheral diameter of the lip. The end plate 11 is provided with an oil leading bore 11a along its axis.

For the bulge forming, a thin tubular material 6 of a length a little shorter than the bulge forming punch 1 is mounted on the bulge forming punch, the both ends of which, thereafter, are inserted into the counterbores of the end plates 11 and 12, which in turn are clamped together by means of bolts 14 extended through opposing holes of the two end plates, or by other suitable means. When high pressure liquid is introduced through the bore 11a of the end plate 11, the hydraulic pressure is exerted equally on the lips 1b, end Walls 1c and inner wall of the central bore 1d, and thus expands the bulge forming punch 1 in radial directions, while simultaneously contracting it in axial direction. In this condition, the lips 1b are pressed fluid-tightly against the walls of the counterbores, whereby the leakage of liquid is prevented. By the radial expansion of the bulge forming punch 1, the thin tubular material 6' is expanded from its inside to follow the convex and concave contour 1a of the bulge forming punch. After the completion of the bulge forming, the hydraulic pressure is released, whereby the bulge forming punch elastically retrieves to initial shape. Accordingly, by removing one of the end plates from the bulge forming punch, the product can be readily taken out.

In the experiment, a seam-welded stainless steel tube of 46 mm. outer diameter and 0.15 mm. thickness was bulged by the bulge forming apparatus described in the above and was readily formed into a product as shown in FIGS. 2a and 217, having convex outer diameter of 51 mm. and concave outer diameter 49.2 mm. The bulge forming apparatus which was used in the experiment is as shown in FIGS. 12a and 12b and 13a and 13b and has the following data: The bulge forming punch 1 is made of a hard polyurethane rubber and has the central bore 1d of 20 mm. diameter, the outer surface incorporating convex portion of 44 mm. outer diameter and concave portion of 42 mm. outer diameter and the lips of 45 mm. peripheral diameter. The end plates 11 and 12 are respectively made of steel and are provided with the counterbores of 44 mm. diameter. The seam-welded stainless steel tube 6' of 46 mm. outer diameter and 0.15 mm. thickness was mounted on the thus prepared bulge forming punch 1, both ends of which, thereafter, were inserted into the counterbores of the end plates 11 and 12, which in turn were clamped together by bolts 14 and nuts 5, as shown in FIG. 12. The bulge forming apparatus thus having been assembled, oil pressure of kg./cm. was applied via the oil leading bore 11a of the end plate 12 for approximately one second. By the application of this internal pressure, the bulge forming punch 1 was elastically expanded radially outward, thus bulging the thin seamwelded stainless steel tube 6' fitted over the bulge forming punch 1 from its inside radially outward according to the convex and concave contour of the outside surface of the bulge forming punch. Thus formed product was in the shape as shown in FIGS. 2a and 2b, wherein the outer diameter of the convex portion was 51 mm. and the outer diameter of the concave portion was 49.2 mm.

In this condition of bulge forming, the side or axial pressure and internal pressures exerted on the bulge forming punch were respectively about 1.00 kg./mm. and the external pressure exerted thereupon was about 0.27 kg./mm. Since these pressures were lower than the elastic limit of the rubber, there occurred no breakage in the bulge forming punch 1. The convex portion of the bulge forming punch elastically reduced its height only a by about 10 percent due to the external pressure of 0.27

kg./mm. Therefore, by giving the bulge forming punch a contour which was correspondingly modified for the elastic deformation, highly precise products could be obtained.

EXAMPLE 3 A bulge forming punch having stepped central bore Still other bulge forming apparatus embodying the present invention is shown in FIG. 14 and a bulge forming punch thereof is shown in FIGS. 15a and 15b. The bulge forming punch 1 in FIGS. 15a and 15b is again made of a hard elastic material such as rubber and is again a thick tube having an outside contour 1a, 1d corresponding to the convex and concave contour of a product article and a stepped central bore 1e, 1 as shown in FIG. 15b, wherein the convex and concave portions 1a and 1d may be of same shape with same diameter, while the central bore portions 1e, 1 are correspondingly formed in different diameters to effect different expanw sions over the length thereof. The other details of this bulge forming apparatus are identical with those of the Example 2.

In the experiment, a seam-welded stainless steel tube 6' of 46 mm. outer diameter and 0.15 mm. thickness was bulged by this bulge forming apparatus and was readily formed into a product as shown in FIGS. 4a and 4b, having the convex and concave portion 1a of convex outer diameter of 51 mm. and concave outer diameter of 49.2 mm. and the convex and concave portion 1d of convex outer diameter of 55 mm. and concave outer diameter of 53.2 mm. The bulge forming apparatus which was used in the experiment is as shown in FIGS. 14 and 15a and 15b and has the following data: The bulge forming punch 1 is made of a hard polyurethane rubber and has the central bore portions 1e of 20 mm. diameter and 1 of 23 mm. diameter, the outer surface incorporating convex portions 1a, 1d of 44 mm. outer diameter and concave portions 1a, 1d of 42 mm. outer diameter and the lips 1b of 45 mm. peripheral diameter and 2 mm. thickness. The end plates 11 and 12 are respectively made of steel and are provided with the counterbores of 44 mm. diameter. The seam-welded stainless steel tube 6 of 46 mm. outer diameter and 0.15 mm. thickness was mounted on thus prepared bulge forming punch 1, both ends of which, thereafter, were inserted into the counterbores of the end plates 11 and 12, which in turn were clamped together by bolts 14 and nuts 15, as shown in FIG. 14. The bulge forming apparatus thus having been assembled, oil pressure of 100 kg./ cm. was applied via the oil leading bore 11a of the end plate 11 for approximately one second. By this process of bulge forming, the stainless steel tube 6' was bulged to a product as shown in FIGS. 4a and 4b, wherein the outer diameters of the convex and concave portions 1a wererespectively 51 and 49.2 mm. and the outer diameters of the convex and concave portions 1b were respectively 55 and 53.2 mm.

In this condition of bulge forming, the side and internal pressures exerted on the bulge forming punch were respectively about 1.00 kg./mm. and the external pressures exerted on the convex portions 1a and 1d were respectively about 0.27 and 0.28 kg./mm. Since these pressures were lower than the elastic limit of the rubber, there occurred no breakage in the bulge forming punch 1. The convex portions 1a and 1d of the bulge forming punch elastically reduced its height only by about percent due to the external pressures of 0.27 and 0.28 kg./mm. Therefore, by giving the bulge forming punch a contour which was correspondingly modified for the elastic deformation, highly precise products could be obtained.

In the above, the three particular examples were explained. However, it will be appreciated that various conditions of bulge formings can be obtained by varying the shape, material, outer diameter and/or hydraulic pressure.

The method of bulge forming of the present invention can be applied to the fields, to which the conventional bulge forming was not applicable. As an example of such applications, the bulge forming of a stator inner liner of a canned motor will be explained in the following.

A completed stator of a canned motor is shown in FIGS. 16a, and 16b and FIG. 17 shows a state in the manufacture where a liner material is freely inserted in its place but not yet fixed. In FIGS. 16a and 16b, a stator core and stator coils 16 are canned or enclosed fluid-tightly within a space defined by an outer shell 17, the inner liner 6 and opposite end plates 18, wherein portions of the inner liner 6 opposing the stator coils 1 6 are formed into corrugation over the circumference. In this kind of stator, the corrugation of the liner 6 and the sealing expansion thereof against the end plates 18 can not be carried out by the conventional method of bulge forming, wherein a split die of a larger diameter than the outer diameter of a work piece is required, such a die, however, being not applicable because of the existence of the coils 16 and the end plates 18. However, according to the present invention, the both ends of the inner liner 6 can be corrugated and expanded for sealing simultaneously and instantly without any requirement of a split die.

Inthe experiment, a seam-welded stainless steel tube of 46 mm. outer diameter, 0.15 mm. thickness and 660 mm. length was readily bulged to a shape having 49 mm. outer diameter of the central straight portion and the convex and concave portions as shown in FIGS. 16a and 16b.

In the following, the results of the experiments to form the stator inner liner by respective bulge forming apparatus are explained.

(i) An example by a bulge forming punch of double structure: FIG. 18 shows the assembled state of a bulge forming apparatus for expanding the central straight portion of a stator inner liner, and the stator inner liner thus formed of its central straight portion is shown in FIG. 19. FIG. 20 shows a state of a further step of manufacture, wherein the partly formed stator is assembled in a bulge forming apparatus employing a bulge forming punch of double structure.

The bulge forming apparatus for forming the central straight portion as shown in FIG. 18 has the following data: The bulge forming punch 10 is made of a highnitril rubber and has a cylindrical portion of 45 mm. outer diameter and 2.5 mm. thickness, which is transferred to opposite end walls of 3 mm. thickness and then to respective inwardly folded portions of 29.5 mm. inner diameter and mm. length. A punch holder 11 is made of steel and has a ventral shaft portion of 35 mm. diameter provided with steppedly reduced portion of 30 mm. diameter at both ends thereof to receive thereon the inwardly folded portions of the bulge forming punch 10. The punch holder is provided with an oil leading 11a of 6 mm. diameter along its axis and a cross bore traversing the central bore 11a. Other fitting members 19, 20, 21, 22, 23, 24 and 25 are all made of steel.

These members were assembled in the following way. At first the bulge forming punch 10 was mounted on the punch holder 11 so that the inwardly folded portions of the bulge forming punch were correspondingly seated on the reduced portions of the punch holder. Then compression rings 19 were arranged at the both ends of the bulge forming punch 8 and were positioned relative to the punch holder by means of positioning sleeves 20 and 21. Thus pre-assembled members were inserted through the stator inner liner material 4 as shown in FIG. 17 so that the bulge forming punch 8 was positioned under the stator core 2, and were clamped with the stator by means of fitting members 12, 13, 14, 15 and 16 as shown in FIG. 18. The members thus having been assembled, oil pressure of 50-100 kg./cm. was applied via the oil leading bore 7a of the punch holder to the inside of the bulge punch 8, which thereby was expanded to urge the liner material 4 against the inner circumference of the stator core 2 to form it as shown in FIG. 19. After holding the hydraulic pressure for about one second, the pressure was released, whereby the bulge forming punch 8 was radially retracted from the inner liner 4 by elastically contracting to its initial shape. Then the fitting members 12, 13, 14, and 16 were unfastened and the inserted bulging assembly was removed.

Subsequently, for the bulge forming of the both end portions of the sleeve material 4', a bulge forming apparatus incorporating the bulge forming punches of double structure was inserted through the partly formed stator and was clamped therewith as shown in FIG. 20. The bulge forming apparatus shown in FIG. has the following data: Bulge forming punches '6 and 7 spacedly arranged therein are made of a hard polyurethane rubber and have central bores of 36 mm. diameter ad the outer convex and concave portions 611 and 7a corresponding to the required shape of the product wherein the outer diameters of the convex and concave portions are respectively 44 and 42 mm. Inner punches 8 and 9 are made of a highnitril rubber and each of them has a cylindrical portion of 35.5 mm. outer diameter and 2 mm. thickness, which is transferred to opposite end walls of 3 mm. thickness and then to respective inwardly folded portions of 23.5 mm. inner diameter and 12 mm, length. Punch holders 11 and 13 are made of steel and each of them has a central portion of 28 mm. diameter provided with steppedly reduced portions of 24 mm. diameter at both ends thereof to receive thereon the inwardly folded portions of the inner punch 8 or 9. The punch holders 11 and 13 are respectively provided with oil leading bores 11 and 13a of 6 mm. diameter along their axes and cross bores respectively traversing the central bores 35a and 37a.

Thus prepared members of the bulge forming apparatus were assembled in the following way. At first, the inner punches 33 and 34 were respectively mounted on the punch holders 35 and 37 so that the inwardly folded portions of the inner punch were correspondingly seated on the reduced portions of the punch holders, and thereupon the bulge forming punches 31 and 32 were correspondingly mounted. Then a pipe threaded portion 35b of the punch holder 35 was screwed into a correspondingly pipe threaded hole of a base plate 38 made of steel. Then the punch holders 35 and 37 were connected via a connector 36 being made of steel and having pipe threaded opposite ends and a central oil leading bore 3611. Thus assembled bulging means were inserted through the partly formed stator inner liner and on one end thereof projecting out of the stator inner liner was mounted an end plate 39 of steel, which was clamped via a washer 40 by a nut 41. At the other end of the stator inner liner, the base plate 38 was clamped to the end plate 18 by bolts 42 and washers 43. Thereafter, oil pressure of 40 kg./cm. was applied through an oil leading bore 38a of the base plate 38 and through the oil leading bores of the punch holder 35, connector 36 and punch holder 37, successively, to spaces 33]) and 34b within the inner punches 33 and 34, whereby the bulge forming punches 31 and 32 were simultaneously expanded in radial directions and the both end portions of the seam- Welded stainless steel tube 6' were simultaneously formed as shown in FIG. 16a. After holding the hydraulic pressure for a few seconds, the pressure was released, whereby the bulge forming punches elastically contracted to their initial shape. Thereafter, by unfastening the nut 41 and the bolts 42, the bulging members could readily be removed out of the formed stator.

(ii) An example by a single bulge forming punch of a simple structure: Another bulge forming apparatus for finishing the both ends corrugation of the stator inner liner as shown in FIG. 16a incorporating a single bulge forming punch according to the present invention is shown in FIG. 21 in the state assembled with the partly formed stator. A bulge punch 1 of this example is made of a hard polyurethane rubber and has central bore of 20 mm. diameter and the outer convex and concave portions 1a corresponding to the required corrugation of the stator inner liner, wherein the outer diameters of the convex and concave portions are respectively 44 and 42 mm. The bulge forming punch is also provided at the both ends thereof with lips 1b of 45 mm. peripheral diameter and 2 mm. thickness, the whole length being longer than that of the stator by 30 mm. This single bulge forming punch was inserted through the partly formed stator inner liner and the both ends thereof were fitted into counter bores of 44 mm. inner diameter and 15 mm. depth of end plates 38 and 39, which in turn were clamped to the end plates of the stator by bolts 42. Thereafter, oil pressure of kg./cm. was applied through an oil leading bore 38a of the end plate 38, whereby the bulge forming punch 1 was expanded in radial directions and the both end portions of the stator inner liner was formed as shown in FIG. 16a. After holding the hydraulic pressure for a few seconds, the pressure was released, whereby the bulge forming punch elastically contracted to its original shape. Thereafter, by removing the bolts 42 and the end plates 38 and 39, the bulge forming punch was readily removed out of the formed stator.

(iii) An example by a bulge forming punch having stepped central bore: The stator inner liner as shown in FIG. 16a could be formed in one process by a bulge forming apparatus incorporating a bulge forming punch having stepped central bore according to the present invention as shown in FIG. 22. The bulge forming punch 1 of this example is made of a hard polyurethane rubber and has substantially same dimensions as the bulge form ing punch shown in FIG. 21, except that the bore is a stepped bore including a central bore portion 1e of 30 mm. diameter to be positioned under the stator core 30 and end bore portions 10. of 20 mm. diameter arranged at both sides thereof. By the bulge forming punch having been applied with oil pressure of 100 l :g./cm. for a few seconds, the corrugated and straight portions of the stator inner liner as shown in FIG. 16a were simultaneously formed.

From the detailed descriptions of the invention in the above, it will be appreciated that according to the present invention the bulge forming is accomplished in an instant by a hollow bulge forming punch of an elastic material having an outer contour corresponding to the required contour of a tubular material to be formed and being inserted into the tubular material without using any outer mould or die, whereby any scarring of the outside surface of the product can be avoided rendering nice appearance of the product and the possibility of application of bulge forming can be greatly broadened to the fields to which the conventional method of bulge forming was not applicable.

I claim:

1. A method of forming a thin tubular material into a required shape, comprising the steps of inserting a bulge forming punch made of a unitary thick tubular body of an elastic material having a non-uniform outer contour into said tubular material which is held substantially unconstrained for free expansion away from the bulge forming punch, said tubular body having an outer contour substantially corresponding to the required shape of the tubular material, and applying fluid pressure to the inside of said bulge forming punch to expand said bulge forming punch in radial directions thereof to such an extent that said thin tubular material is expanded by said bulge forming punch beyond the limit of elastic deformation thereof to follow the outer contour of the expanded bulge forming punch.

2. A method according to claim 1 wherein said tubular body has a substantially uniform mean tube thickness along the axial length therof, whereby said tubular material is expanded substantially uniformly along the axial length thereof.

3. A method according to claim 1, wherein said tubular body has portions of different mean tube thicknesses along the axial length thereof, whereby said tubular material is expanded to different mean diameters along the axial length thereof.

4. An apparatus for forming a thin tubular material into a required shape, comprising a bulge forming punch made of a unitary thick tubular body of an elastic material adapted to be inserted into a tubular material and having a non-uniform outer contour substantially corresponding to a required shape of such tubular material, and means for applying fluid pressure to the inside of said bulge forming punch to expand said bulge forming punch in radial directions thereof to such an extent that said thin tubular material is expanded by said bulge forming punch beyond the limit of elastic deformation thereof to follow the non-uniform outer contour of the expanded bulge forming punch.

5. An apparatus according to claim 4, wherein said tubular body has a substantially uniform mean tube thickness along the axial length thereof so that said tubular material is expanded substantially uniformly along the axial length thereof.

6. An apparatus according to claim 4, wherein said tubular body has portions of different mean tube thicknesses along the axial length thereof so that said tubular material is expanded to different mean diameters along the axial length thereof.

7. An apparatus according to claim 4, wherein said tubular body has inclined annular lips at both axial ends thereof which are adapted to be inserted into counterbores of cooperating punch holding means for keeping fluid pressure applied to the inside of said bulge forming punch.

8. An apparatus for forming a thin tubular material into a required shape, comprising a bulge forming punch made of a unitary thick tubular body of an elastic material adapted to be inserted into said tubular material and having a non-uniform outer contour substantially corresponding to a required shape of the tubular material, a tube means made of an elastic material having substantially the same axial length as said bulge forming punch and adapted to be inserted into said bulge forming punch, and means for applying fluid pressure to the inside of said tube means to expand said tube means and said bulge forming punch in radial directions thereof to such an extent that said thin tubular material is expanded by said bulge forming punch beyond the limit of elastic deformation thereof to follow the outer contour of the expanded bulge forming punch.

References Cited UNITED STATES PATENTS 2,557,722 5/1951 Bra-uchler 72393 X 2,865,640 12/1958 Watson et al 7254 X 3,357,229 12/1967 Luedi 7259 X 2,825,387 3/1958 Alltop et a1. 7259 2,581,787 1/1952 Dreyer 7259 3,379,805 4/1968 Roberts 26494 FOREIGN PATENTS 350,946 2/1961 Switzerland.

CHAR-LES W. LANHAM, Primary Examiner A. L. HAVIS, Assistant Examiner U .5. Cl. X.R. 

